ГОДИШНИК НА МИННО-ГЕОЛОЖКИЯ УНИВЕРСИТЕТ “СВ. ИВАН РИЛСКИ”, Том 57, Св. II, Добив и преработка на минерални суровини, 2014
ANNUAL OF THE UNIVERSITY OF MINING AND GEOLOGY “ST. IVAN RILSKI”, Vol. 57, Part ІI, Mining and Mineral processing, 2014
LOW TEMPERATURE SYNTHESIS OF SILVER NANOPARTICLES
Georgi Nikolov, Maria Manastirli, Gospodinka Gicheva
University of Mining and Geology “St. Ivan Rilski”, 1700 Sofia,
ABSTRACT. Metal nanoparticles are of great importance for modern science. With their numerous applications in many different areas of technology they have attracted a great deal of interest in their direction. Nobel metals nanoparticles such as silver (Ag NP) and gold (Au NP) in particular have been intensively studied due to their stability and interesting and unique physical and optical properties. Due to them they have been engaged in wide areas of research including photocatalysis and biotechnology. This fact is pushing scientists into developing new and cost effective methods for metal nanoparticle synthesis. The key component here is “cost effective”. If metal nanoparticles are to be applied in large scale production their preparation have to be relatively cheap. Here we present a low temperature synthesis of silver nanoparticles which is a precondition for their potential large scale production.
Keywords: nanotechnology, metal nanoparticles, synthesis.
НИСКОТЕМПЕРАТУРЕН СИНТЕЗ НА СРЕБЪРНИ НАНОЧАСТЦИ
Георги Николов, Мария Манастирли, Господинка Гичева
Минно-геоложки университет “Св. Иван Рилски”, 1700 София,
РЕЗЮМЕ. Металните наночастици са от излючително значение за съвременнта наука. Наночастиците от благородни метали, в частност сребро и злато, биват изследвани интензивно поради тяхната стабилност и уникални физични и оптични свойства. Именно заради тези си свойства те биват прилагани в различни области на науката, като фотокатализа и биотехнологии. Именно този широк спектър на приложение на наночастиците кара учените да разработват нови и икономически изгодни методи за синтез с оглед потенциалното им промишлено производство. Ако искаме да разработим метод за производство на метални наночастици в индустриален мащаб, то тяхната цена трябва да е относително ниска. В настоящото изследване се предлага такъв евтив нискотемпературен синтез на сребърни наночастици, което е необходима предпоставка за тяхното производство в промишлени мащаби.
Ключови думи: нанотехнологии, метални наночастици, синтез
1
Introduction
Metal nanoparticles have attracted the attention of researchers due to their unique optical and electronic properties (Liz-Marzán, 2006). Their applications vary in different areas of modern technology – optics and electronics (Azizi-Toupkanloo H et al, 2013; Saion et al, 2014), catalysis (Philip et al., 2014; Ehsani et al., 2014; Janani, 2014), biology (Abdel-Wahhab M., 2014), medicine (Bandara et al., 2013, Çeliköz et al., 2005) and etc. Large part of the interest in them stems from the fact that they possess an antimicrobial activity. In the recent years due to the extensive overuse of antibiotics a generation of antibiotic resistant strains (and infections) emerged that can’t be easily treated by standard procedures. In response to the growing concern people have turned to an age-old cure – silver. Its antibacterial properties have been long known. They are attributed to the silver ions (Ag+) but with the development of nanotechnology and the production of silver nanoparticles (Ag0) it was established that they also possess such activity. There are even commercially available pharmaceutical products containing silver nanoparticles (Colargol®, Chemax Pharma), presently. So incorporating silver nanoparticles gives us an opportunity to produce materials and devices with antibacterial activity such as antibacterial textile, antibacterial wound bandages, vials and containers with antibacterial surfaces as well as water treatment devices. Other major field of application exploits some of the physical properties of the silver nanoparticles (AgNPs) like their conductivity. They are used in production of inkjet inks for fabrication of flexible electronic displays via inkjet printing technology (Abe et al, 2011).
Having said that, it becomes apparent that low cost and environmental friendly (“green”) method for silver nanoparticles synthesis is key component for their mass production. Some of the most common methods for AgNPs production involve chemical reduction of silver salt (silver nitrate AgNO3 for example) by various reductors such as sodium borhydride (NaBH4), sodium citrate (Na3Cit), formaldehyde and some saccharides – glucose, galactose, lactose and maltose (Zboril et al, 2006). The chemical reaction that takes place results in formation of silver nanoparticles (AgNPs). Even though the process itself is easily conducted with high yield, a control over the size of the obtained nanoparticles remains elusive. It should be pointed out that size control for this kind of material is of great importance since all properties of nanomaterials are size dependent. It was found that the antimicrobial activity of silver nanoparticles strongly depends on their size and size distribution. So one can see how important is to come up with a synthesis method which could allow us a control over the size of the AgNPs.
Other methods involve reduction of silver ions in the presence of amines (Abe et al, 2011). These methods have their merits based on the low temperature of the synthesis process and control over the size of the obtained AgNPs. Following that examples a low temperature method for silver nanoparticles has been investigated. It has the benefit of being cost effective using relatively inexpensive reagents, nontoxic organic solvents and lacking the need of heating the reaction mixture thus saving additional expenses for production. These features allow the method to be potentially applied for industrial production of AgNPs.
Experimental part
Materials
Silver nitrate (AgNO3) and trisodium citrate (Na3Cit.2H2O) were purchased by Merck KGaA, triethanolamine (TEA; ≥98%) by Sigma-Aldrich and polyvinylpyrrolidone (PVP K30; Mw ~40 000) by Fluka. All reagents were used without any further purification or modification.
Synthesis of silver nanoparticles (AgNPs)
A definite amount of PVP (0.5 g) was dissolved in distilled water under magnetic stirring at room temperature until a clear solution was obtained. A silver nitrate salt (0.25 g) was then added to this solution under constant stirring to its total dissolve. After 10 min an aqueous solution of sodium citrate (0.44 g) was added dropwise to the mixed solution of PVP and AgNO3. After the gradual addition of sodium citrate (Na3Cit) the solution slowly turns opaque. We proceeded by adding a solution of triethanolamine (TEA) to the reaction mixture and then the colour turned from white to light brown signifying the formation of silver nanoparticles.
Materials characterization
UV-VIS absorbance spectra of the AgNPs dispersed in water were taken on UV-VIS spectrophotometer Shimadzu UV160U. Atomic force microscopy analysis of selected samples was performed.
Results and discussion
The AgNPs synthesis method presented in this research is conducted at room temperature. It allows us to obtain nanoparticles without heating the reaction mixture. In order to gain some insight on the process of formation and the factors affecting it we have conducted series of experiments varying some of the reagents concentration (Table 1). As an indication of the size of the AgNPs we take into account the wavelength of the maximum in the absorbance spectrum (Liz-Marzán, 2006). It is considered to be indicative for the silver nanoparticles’ size at same other conditions, so AgNPs with longer wavelength maximum are generally considered larger.
Table 1
№ / AgNO3, g / PVP, g / Na3Cit, g / TEA, ml / Λmax, nm1 / 0.25 / 0.5 / 0.44 / 0.020 / 420
2 / 0.25 / 0 / 0.44 / 0.020 / 420
3 / 0.25 / 0.5 / 0.44 / 0.060 / 415
4 / 0.25 / 0 / 0.44 / 0.060 / 410
5 / 0.25 / 0.5 / 0.44 / 0.180 / 410
6 / 0.25 / 0 / 0.44 / 0.180 / 405
Effect of PVP
In the series of conducted experiments we altered the amount of polyvinylpyrrolidone (PVP) in order to establish its effect on the formation of silver nanoparticles. From the data presented in Table 1 becomes apparent that changing the amount of PVP (from 0 g in sample “2” to 0.5 g in sample “1”) does not greatly affect the final size of the obtained nanoparticles, but not is not the case of samples marked as “3”, “4”, “5” and “6”. We observed that in the cases where the concentration of TEA is higher (“3”, “4”, “5” and “6”) there is distinction between the samples with and without PVP. We can see that where PVP is present in the reaction mixture the final size of the AgNPs is bigger. Some literature sources claim that a coordinative bond between silver ions (Ag+) and N and/or O atoms form PVP molecule is formed resulting in the formation of a complex (Abe et al, 2011) with increased reactivity compared to silver ions alone. That could explain why in the series with higher concentration of TEA, which acts as a catalyst, the more active PVP-Ag+ complex will give AgNPs with bigger size.
Other important distinction between the reaction mixtures containing PVP is the fact that the obtained dispersions of AgNPs in water are much more stable throughout the time compared to the ones without PVP. PVP acts as surfactant stabilizing the dispersion most likely by coordinating to the surface of the obtained AgNPs as some sources indicate (Abe et al, 2011).
Fig. 1. Absorbance spectra of silver nanoparticles AgNPs obtained at different reaction conditions (Table 1)
The absorbance spectra of AgNPs dispersions were taken (Fig. 1). The spectra exhibit a maximum in the region 405-420 nm characteristic for them and in good agreement with the literature data for silver nanoparticles. The position of absorbance maximum in spectrum of the different samples depicts the difference in their size.
The effect of PVP on AgNPs morphology will be further investigated by transmission electron microscopy (TEM) to see if there is any difference in the shape of the resulting silver nanoparticles.
Effect of TEA
One factor that significantly affects the formation of AgNPs is the amount of the added amine. Looking at Table 1 one can clearly see that increasing the amount of the amine (TEA) in reaction mixture leads to the formation of smaller AgNPs. That bodes well with the literature data (Abe et al, 2011) of other authors that have observed the same effect. It is proposed that TEA act as some sort of catalyst of the process of reduction of silver ions to silver nanoparticles by Na3Cit thus increasing its rate. When AgNPs are produced by reduction of silver ions by citrate alone (“citrate method”) the reaction mixture needs to heat up to 1000C in order for reaction to take place. When adding amine (TEA) reaction proceeds at room temperature. Since it was suggested that TEA takes part in the reaction, increasing its concentration will lead to faster reaction rate and smaller size of the resulting AgNPs. This assumption is supported by the fact that the size of the obtained AgNPs is smaller indeed (Figure 1).
The effect of amine (TEA) concentration on AgNPs morphology and size distribution is yet to be thoroughly investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS) to confirm the results obtained by UV-VIS spectrophotometry. The additional results will broaden our view in the process of AgNPs formation and to give us a clearer perspective about the nature of the process.
Conclusions
In this study a method for low temperature synthesis of AgNPs is discussed. In involves addition of amines to the “citrate method” which allows the reaction to take place at room temperature. Some of the main advantages of the method include the use of low cost reagents, omitting usage of toxic organic solvents and lacking the need of heating up the reaction mixture. It was shown that by varying the amount of amine the size of the obtained AgNPs can be controlled. All of the above mentioned facts suggest that this method could be employed in industrial production of AgNPs.
Acknowledgements
This research was funded by project MTF-133/ 09.05.2014 of UMG “St. Ivan Rilski”.
References
Abdel-Aziz M.S., Shaheen M.S., El-Nekeety A.A., Abdel-Wahhab M.A. 2014, Antioxidant and antibacterial activity of silver nanoparticles biosynthesized using Chenopodium murale leaf extract. – In: Journal of Saudi Chemical Society, 18(4), 356-363.
Anbazhagan V., Ahmed K.B.A, Janani S. 2014, Synthesis of catalytically active silver nanoparticles using lipid derived signaling molecule, N-steroylethanolamine: Promising antibacterial agent and selective colorimetric sensor for mercury ion. – In: Sensor Actuat B-Chem, 200, 92-100.
Faraji N., Yunus W.M.M, Kharazmi A., Saion E. 2014, Third-order nonlinear optical properties of silver nanoparticles mediated by chitosan. – In: Optik - International Journal for Light and Electron Optics, 125(12), 2809-2812.
Goharshadi E.K., Azizi-Toupkanloo H. 2013, Silver colloid nanoparticles: Ultrasound-assisted synthesis, electrical and rheological properties. – In: Powder Technol, 237, 97-101.
Liz-Marzán L.M. 2006, Tailoring surface plasmons through the morphology and assembly of metal nanoparticles. – In: Langmuir, 22(1), 32-41.
Nasrollahzadeh M., Babaei F., Sajadi S.M., Ehsani A. 2013, Green synthesis, optical properties and catalytic activity of silver nanoparticles in the synthesis of N-monosubstituted ureas in water. – In: Spectrochim Acta A, 132, 423-429.
Natsuki J., Abe T. 2011, Synthesis of pure colloidal silver nanoparticles with high electroconductivity for printed electronic circuits- The effect of amines on their formation in aqueous media. – In: J Colloid Interf Sci, 359, 19-23.
Panacek A., Kvitek L., Prucek R., Kolar M., Vecerova R., Pizurova N., Sharma V.K., Nevecna T., Zboril R. 2006, Silver nanoparticles: Synthesis, characterization, and their antibacterial activity. – In: J. Phys. Chem. B, 110, 16248-16253.
Perera S., Bhushan B., Bandara R., Rajapakse G., Rajapakse S., Bandara C. 2013, Morphological, antimicrobial, durability, and physical properties of untreated and treated textiles using silver-nanoparticles. – In: Colloid Surface A, 436, 975-989.
Ülküra E., Oncul O., Karagoz H., Yeniz E., Çeliköz B. 2005, Comparison of silver-coated dressing (Acticoat™), chlorhexidine acetate 0.5% (Bactigrass®), and fusidic acid 2% (Fucidin®) for topical antibacterial effect in methicillin-resistant Staphylococci-contaminated, full-skin thickness rat burn wounds. – In: Burns, 31(7), 874-877.
Vidhu V.K., Philip D. 2014, Spectroscopic, microscopic and catalytic properties of silver nanoparticles synthesized using Saraca indica flower. – In: Spectrochim Acta A, 117, 102-108.
The article has been reviewed by assoc. prof. N. Mincheva-Peneva and recommended for publication by department “Chemistry”.
1